The Demonic Hell Bearers are colossal demons originating straight from the depths of Hell. They participated in the siege of Bastion's Keep under Azmodan's army.
"I have long hoped that the demonic hell bearer was a fiction, a nightmare from the troubled minds of my ancestors. Just imagine a demon taller than a castle tower, who can retch up endless minions from the depths of the Hells! Surely our people are doomed if such an abomination exists."— Deckard Cain(src)
The Hell Bearers look much like giant centipedes, clinging upon the walls and anchoring with their claws. Once they are in position, they start to regurgigate Demon Troopers, who pour forth from the Hell Bearer's maw in packs of four at a time. The total number of demons each Hell Bearer can transport is finite (except for the Siegebreaker fight), but killing the demon will end the reinforcements, as the living siege tower will lose its grasp and fall to the ground below.
The Hell Bearers are quite durable, but may not attack directly. Once out of demons to vomit, they are no longer dangerous and can be finished off in any way players like.